This invention relates to an optical filter and method of making an interferometric optical filter by irradiating one or more substantially similar waveguide arms with light of an intensity, wavelength and duration sufficient to vary the optical path length difference between the two arms to obtain a desired output response.
Mach-Zehnder Interferometers (MZIs) are well-known devices in planar waveguide technology and in optical fibers. An MZI is comprised of two couplers having a phase shifting region therebetween which consists of two arms, 10a and 10b shown in FIG. 1, having differential propagation constants. Unbalancing the MZI, and more specifically the arms of the MZI produces a sinusoidally varying spectral output behavior as is shown in FIG. 5a useful in a number of filtering or gain shaping applications. In planar waveguides, such as those made in lithium niobate or poled polymer materials, the application of a voltage and use of the electro-optic effect is used to controllably unbalance the MZI. Fiber-optic based MZI""s are often unbalanced by using different fiber lengths in each arm, using dissimilar fibers in the two arms such as fibres having different core radii, or by varying the core dimension by using a bi-conical taper in one of the arms. Notwithstanding, these solutions have their limitations; for example, different fiber lengths are undesirable because of packaging difficulties and a large temperature sensitivity. The use of dissimilar fibers have an unwanted polarization and wavelength sensitivity. Furthermore, the use of dissimilar fibres lessens the amount of control afforded in the manufacture process. The use of a fused biconical taper in one of the interferometer arms introduces an unwanted wavelength sensitivity.
In 1989 researchers at the Communications Research Center (CRC) in Ottawa, Ontario, Canada have shown that an unbalanced MZI can be made using dissimilar fiber fused taper couplers. In particular, the optical path lengths of the two arms are different because the two fibers fused together to make the MZI are dissimilar and, therefore, have different propagation constants (B. Malo, F. Bilodeau, K. O. Hill, D. C. Johnson and J. Albert, Electronics Letters, Oct. 12, 1989, Vol. 25, pp. 1416-1417). Then, in 1990 the same researchers at CRC used an unbalanced MZI with dissimilar fibers as a measurement tool for the purpose of measuring the ultraviolet light photosensitivity in germanium-doped silica fiber. This was described by B. Malo, K. A. Vineberg, F. Bilodeau, D. C. Johnson and K. O. Hill, in Optics Letters, Sep. 1, 1990, Vol. 15, pp. 953-955.
There are a number of limitations of using dissimilar fibers in an MZI. First, a residual polarization dependence and polarization dependent loss can result from the use of dissimilar fibers. Another problem is that the couplers can vary with frequency, limiting the wavelength range over which the device operates accurately. Also, the only tunable parameter during the MZI manufacturing process given two dissimilar fibers is the position of the tapers.
In contrast, using the same fiber in both arms of the MZI reduces the polarization dependence and increases the wavelength range of operation.
As an improvement to the measurement system, the CRC researchers have also used an MZI made from the same fiber. However, an MZI made from identical fibers of equal arm length is not useful for the measurement of small optical phase shifts because it is balanced. In order to create an optical path-length difference or imbalance between the two interferometer arms, they used a novel design in which one of the fibers is pre-tapered in the middle over a 13-mm length, reducing the core-mode effective index in the tapered region (F. Bilodeau, D. C. Johnson, B. Malo, K. A. Vineberg, K. O. Hill, T. F. Morse, A. Kilian and L. Reinhart, Optics Letters, Oct. 15, 1990, Vol. 15, pp. 1138-1140). Thus, a symmetric MZI I purposefully tapered in one arm of the MZI to unbalance it. This unbalanced MZI was then used to measure refractive index changes caused by light exposure by irradiating the untapered arm.
In summary, unbalanced MZIs have been made and used for the purpose of measuring the photosensitivity in one of the waveguides. Unbalancing was achieved by using different optical fibres and an another instance by introducing a taper in one of the arms thereby reducing the core-mode effective index in the tapered region.
Hermetically sealed MZI couplers are known to have good environmental stability by inserting two dissimilar fibers in a glass tube. In particular, these have been fabricated by inserting fibers with differing core diameters and index deltas into a glass tube doped with barium oxide. The tube is then heated and collapsed on the fibers. The collapsed tube is then drawn to provide for the two couplers. The magnitude of the coupling can be easily adjusted by changing the taper length as descried by D. A. Nolan, W. J. Miller, R. Irion, Optical Fiber Conference, February 1998, San Jose, Calif., OFC""98 Technical Digest, pp. 339-340. Fiber-based lattice devices have also been made by cascading different MZI""s. These lattice filters are made by using three unequal couplers surrounding two different phase shifting regions (D. A. Nolan, IEEE International Passive Components Workshop, September 1998). In addition, using a series of MZI""s can lead to filter synthesis using Fourier expansion for applications in band splitters and combiners and gain equalization (Y. P. Li and C. H. Henry, Optical Fiber Telecommunications IIIB, eds. I. P. Kaminow and T. L. Koch. San Diego: Academic Press, 1997, Ch. 8, pp. 345-351).
The great amount of interest in wavelength division multiplexing (WDM) system applications has caused a surge in devices based on fused biconically tapered (FBT) fiber couplers. In general, the technologies used to produce passive components divide them into three main categories: integrated optics, micro-optics and all-fiber devices. FBT couplers are suitable for low-cost, low port count applications. Planar integrated-optic components are ideal for network branching applications requiring 16, 32 or higher port count. However, micro-optic couplers and wavelength splitters are more suitable for high-performance systems (R. Chua, FiberOptic Product News, November 1998, pp. 31-34).
In its simplest form, a FBT fiber coupler consists of two optical fibers whose optical cladding has been fused together. The structure is tapered by elongation while it is hot until appropriate coupling properties are achieved. It is then bonded to a substrate and encapsulated into a compact and rugged package. This is shown in FIG. 2. Unfortunately, FBT""s are polarization dependence and consequently exhibit polarization dependent loss (PDL) in a system. As in any two-path interferometer, the optical path difference must be large to yield high spectral selectivity. However, since the fiber coupler is a birefringent structure, longer length leads to greater polarization dependence. The FBT coupler of FIG. 2 behaves much like the MZI of FIG. 1, where the optical path difference between the two arms results in a sinusoidal wavelength response. However, unlike the FBT the MZI has a very small polarization dependence and PDL because the couplers themselves in FIG. 1 are very short (F. Gonthier, FiberOptic Product News, September 1998, pp. 54-56).
Recently UV-induced fiber Bragg gratings have been making a tremendous impact on fiber-optic communications. The gratings an be created directly in the germanium-doped core of optical fibers by holographic interference techniques, using phase or amplitude masks, or by point-by-point writing techniques (c.f. .A. E. White an S. G. Grubb, Optical Fiber Telecommunications IIIB, eds. I. P. Kaminow and T. L. Koch. San Diego: Academic Press, 1997, Ch. 7, pp. 273-276). At first, the photosensitivity of ordinary transmission fiber was too weak to write the strong gratings of interest for applications. The invention of a sensitization process called hydrogen loading in 1993 made it possible to write useful gratings in standard fiber, enabling a host of practical applications (P. J. Lemaire, R. M. Atkins, V. Mizrahi and W. A. Reed, Electronics Letters, Vol. 29, pp. 1191-1193). In this process, the fiber is exposed to high-pressure (20-750 atm) hydrogen or deuterium at moderate temperatures (21-75 C.) for up to a week. Hydrogen loading makes a germanium-doped fiber controllably photosensitive. Without the loading, index changes that have been observed are on the order of 10xe2x88x924. With hydrogen loading, index changes as large as 10xe2x88x922 have been achieved. The unreacted hydrogen diffuses out during a subsequent annealing process.
Thus, whereas much work was done on FBT couplers and MZI in the 1980""s, much of the focus on new devices in the 1990""s has centered around fiber Bragg gratings. What has changed since the 1980""s and early 1990""s is that much larger index changes can now be induced in optical fibers through light exposure.
Thus with the advance of enhancing the photosensitivity of optical fibres it becomes possible and practicable to use light exposure in new and previously impracticable uses. For example, in accordance with this invention, light exposure can be used to unbalance symmetric MZI""s for practical applications in filters, gain flattening and equalization elements, and band splitters and combiners in accordance with this invention.
It is an object of the invention to use light exposure to unbalance a symmetric MZI structure made from substantially the same fiber of substantially the same length. In particular, it is an object of this invention to unbalance a substantially symmetric MZI by exposing at least one of the arms of the MZI to light to change the index of refraction.
It is object of the invention to manufacture environmentally stable and mechanically tunable MZI""s with substantially identical fibers in physical contact. Physical contact of the fibers prevents differential bending and differential thermal fluctuations between the cores. In addition, these components can be tuned in a controllable fashion by bending the phase-shifting regions between the couplers or the couplers themselves.
It is yet object of the current invention to make fiber-based MZI with two coupler regions surrounding two or more substantially same fibers of substantially the same length with unbalancing occurring through the use of a means of altering the index of refraction of at least one fiber arm.
It is another object of this invention to induce an index change xcex94n over at least a section of one arm of the fiber-based MZI Lexp such that the frequency spacing between peak and null for the transmission through the device is given by xcex94f=c/(2 xcex94n Lexp).
It is another object of this invention to accurately tailor the spectral response of the MZI device by monitoring the MZI with a broadband light source and an optical spectrum analyzer during the light exposure process. The precise amount of differential phase shift can be controlled by adjusting the exposed fiber length, the exposure time, the intensity of the light, the degree of hydrogen loading of the fiber and the doping of the fiber.
It is another object of the current invention to increase the yield during the manufacturing process by using light exposure within some length of the remaining arm of the MZI to trim and compensate for any overexposure during the first light induced index change stage in the first arm.
It is yet another object of this invention to cascade the light-exposed unbalanced MZI so as to create lattice devices such as Lyot-Ohman filters or Solc filters for applications such as gain equalizers or band splitters. The light exposure in different sections of the MZI""s can be varied to vary the phase shift in each section, and the coupling ratios of the couplers used in the cascade can be tailored to achieve the desired transmission function.
It is another object of the current invention to cascade MZI to make Fourier filters. A major advantage of using light exposure to unbalance symmetric MZI""s is the flexibility of changing the index in either the top or bottom arms in cascaded devices. Irradiating either the top or bottom arms of the MZI can create both positive and negative phase shifts. The phase shifts and coupling ratios can also be varied in each stage of the Fourier filter.
Moreover, it is an object of the current invention to use light exposure or other index altering means to unbalance a symmetric MZI made in planar waveguide structures.
Finally, it is another object of the current invention to use light exposure or other index altering means in more general fiber-based or waveguide interferometric structures such as Michelson or Sagnac interferometers.
The advantages gained through the current invention over the prior art are:
The MZI in accordance with this invention exhibits a low polarization sensitivity and low polarization dependent loss since substantially the same fiber is used in both arms. Any remaining polarization sensitivity comes from the couplers. However, this sensitivity will still be much lower than for fused taper couplers;
The MZI in accordance with this invention operates optimally over a wider wavelength or frequency range, since the same fiber is used in both arms, the two arms track each other in wavelength;
The MZI in accordance with this invention has low environmental sensitivity since substantially the same fibers of substantially the same length are used in physical contact with one-another;
The quality control during the manufacturing process of the MZI in accordance with this invention is high through appropriate monitoring and control of the fabrication parameters during the light exposure stage. In addition, the yield of the manufacturing can be high since the second arm of the MZI can be light exposed to trim and compensate for any overshoot during the original light exposure step; and,
Fourier filters, which require that either the top or bottom arm of the MZI be light exposed, can uniquely be made through the current invention by shifting the light exposure between the two arms of the MZI.
In contrast, it is much harder to make Fourier filters from MZI""s consisting of two dissimilar fibers; and, Loop mirrors or Sagnac mirrors can be made that maintain the state-of-polarization by using light exposure to induce birefringence in the fiber or waveguide;
In accordance with the invention, there is provided, a method of making an interferometric optical filter comprising the steps of:
a) providing a splitter and combiner;
b) providing two arms coupled to the splitter and combiner, the two arms each having a core and a cladding, the arms being substantially same optical path length and having substantially similar mode field diameters along the length thereof; and,
c) irradiating a region of one of the two arms with light of a suitable intensity and duration so as to vary the refractive index n over a region of length L in a substantially uniform manner of said one of the two arms to provide a refractive index difference xcex94n and an optical path length difference between the two arms to effect a change xcex94f in the sinusoidal spectral response of the filter and, where xcex94f is the spacing between peak and null wavelengths, such that Lxcex94n=c/2xcex94f.
In accordance with the invention there is further provided, a method of making an optical filter comprising the steps of:
a) providing an MZI having two substantially similar arms disposed between two optical couplers, the arms each having a core and a cladding and having a substantially same optical path length;
b) irradiating one of the two arms with light of a suitable intensity and duration so as to vary the refractive index of an irradiated region and to provide a difference in the optical path lengths of the two arms such that light divided by one of the couplers propagating along the two arms will be phase shifted when combined by the other of the couplers.
In accordance with the invention, there is further provided, a method of making an optical filter comprising the steps of:
a) providing an optical coupler for splitting and coupling light to or from two arms, the two arms being substantially similar in mode field diameter, the arms each having a core and a cladding and having a substantially same optical path length;
b) irradiating a region of at least one of the arms of the two arms with light of a suitable intensity and duration so as to vary the refractive index of the irradiated region substantially uniformly to vary the optical path length of at least one of the two arms and to provide a phase offset for light propagating within the two arms.
In accordance with another aspect of the invention, there is further provided, an optical filter comprising:
an optical coupler for splitting and coupling light;
two unbalanced optical waveguide arms directly coupled with the optical coupler, the arms being substantially similar along their length other than having a dissimilarity caused by a refractive index change in at least one light irradiated region in at least one of the waveguides providing a difference in optical path length and refractive index between the arms that would otherwise not exist in the absence of light irradiation of at least one of the arms.
In accordance with this invention, it is preferred that the pair of waveguides for example in a Mach Zehnder interferometer disposed between two couplers are as identical as possible prior to the irradiating step. Of course, it is understood that substantially similar includes variations in mode field diameter, in some instances of up to 30%. Notwithstanding, it is preferred to have a smaller dissimilarity.